Albert Barnes on John 16:30

Now are we sure that thou knowest, &c. Their difficulty had been to understand what was the meaning of his declaration in Jn 16:16. About this they conversed among themselves, Jn 16:17-19. It is evident that they had not mentioned their difficulty to him, and that he had not even heard their conversation among themselves, Jn 16:19. When, therefore, by his answers to them (Jn 16:20-28), he showed that he clearly understood their doubts; and when he gave them an answer so satisfactory without their having inquired of him, it satisfied them that he knew the heart, and that he assuredly came from God. They were convinced that there was no need that any man should ask him, or propose his difficulties to him, since he knew them all and could answer them.